This invention relates to conversion of cellulosic plant wastes to useful products. More particularly, it relates to methods of binding particles of silica-rich plant materials into a unitary structure and to products produced by such methods.
The manufacture of composition board products (sometimes referred to as engineered wood products) has become widespread and commonplace, primarily as a result of efforts to reduce consumption of exhaustible timber. As trees are harvested and millwork ensues, the residual bark, chips and sawdust are used to create board products for consumer applications. Such millwork reclaimed products serve many consumer needs while reducing consumption of exhaustible trees. These products are known as oriented strand boards, clipboards, particleboards and medium and high density fiberboards and are commonly used for (but not limited to) shelving, furniture, flooring, paneling, cabinets, doors, roofing, underlayment and sheathing construction materials.
Composition wood products are ordinarily formed using resins, ureas, phenols and formaldehyde. Often these products must be further processed with laminations and various structural and cosmetic treatments for consumer use and acceptance. While these manufactured products offer economic and conservation advantages and exhibit several characteristics of solid wood products, they are generally limited in application and are formed using chemical binders with undesirable and adverse environmental and health impacts. Furthermore, most technologies associated with formation of engineered wood products require reduction of wood to usable fiber form; employ wet chemistries; and require high temperatures and high pressures. Thus, they require high energy consumption and raise environmental issues relating to disposal of process wastes and waste water reclamation and clarification.
The presence of high levels of silica in the rice plant is well documented and has limited use of this plant fiber in commercial applications. For example, when burned for cogeneration, high levels of residual build-up may cause physical damage to incineration devices. Silica-rich plant material is slow to decompose, thus creating nutrient depletion of soil and elevated microbial growths which are detrimental to soil. As a feed filler, the silica manifests a degree of hardness and insolubility which interferes with ingestion and digestion. Since there is a reluctance to bond, the individual hulls have been characterized as shifting, thus reducing structural soundness when used as a filler for concrete, asphalt, etc.
In accordance with the present invention, substantially dry particles of silica-rich plant materials are bonded into a unitary product to produce structural material similar to fiber materials and particleboards without use of conventional resins or the like. The term xe2x80x9csilica-richxe2x80x9d is used herein to define cellulosic materials of plant origin which comprise approximately eight percent (8%) or more by weight silica Rice hulls and stalks are typical of such silica-rich materials.
The invention includes using a polymeric resin base material preferably comprising 4,4xe2x80x2 or 2,4xe2x80x2 methylene diisocyanate in the range of from about 20% to about 55% by weight and a functionality greater than about 2. The preferred polymeric resin base material is PAPI or similar diisocyanate containing polymeric by-products obtained as a result of the manufacture of MDI.
This invention also relates to the modification of TDI, MDI, HDI, NDI, and any derivatives thereof. More particularly, it relates to a method of incorporating derivatives of silicon into resin blends including but not limited to any variety of diisocyanates associated with prepolymeric or polymeric materials, polyols, and polyisocyanates to increase adhesive bond strength potential. Additionally, as the silicon content is increased, the modified resin is increasingly tolerant to dilution by water wherein adhesive bond strength potential is extended to produce suitable product for commercial applications at low cost.
The invention includes a method for preparing a binder material for cellulosic products which comprises providing a polymeric resin base material preferably comprising 4,4xe2x80x2 or 2,4xe2x80x2 methylene diisocyanate in the range of from about 20% to about 55% by weight and a functionality greater than about 2, forming a colloidal gel from silica and a basic solution, and incorporating the colloidal gel into the polymeric base material at a temperature, rate of addition and agitation level to achieve a substantially homogeneous binder material. The preferred polymeric resin base maternal is PAPI or similar diisocyanate containing polymeric by-products obtained as a result of the manufacture of MDI. The colloidal gel is preferably formed by addition of an aqueous solution of an alkali metal hydroxide to solid silica. The amount of colloidal silica incorporated into the polymeric base resin is preferably in the range of from about 40% to about 60% of the molecular weight of the NCO content of the base polymer.
In another aspect, the invention relates to a method for increasing the binding capability and/or stability of diisocyanate containing polymeric resins, some of which have heretofore been considered by-products of no, or limited, utility in the bonding of cellulosic products. By modifying such compositions with silica based materials a new binder material is formed which displays enhanced binding capabilities and stability as compared to the unmodified base resin material. Such new binders can be further diluted with water and used to economically produce bonded cellulosic structures useful in a wide range of applications.
In the preferred embodiment of the invention, a predetermined quantity of silica-rich plant material such as dry rice hulls and/or comminuted rice stalks is weighed and placed in a mixing vessel. No moisture need be added to effect successful binding, the moisture content of the silica-rich plant material itself being sufficient. The term xe2x80x9csubstantially dryxe2x80x9d as used herein refers to silica-rich plant materials that has no additional water added to it, but may contain natural amounts of water ranging up to 10% by weight. The binder material is added to attain a concentration by solids of about 3-20%. However, concentrations higher than 10% appear excessive and sometimes cause subsequent leaching and solidification outside the corpus of the rice hulls. The ingredients are then thoroughly mixed to insure distribution of the base resin material into the dry plant matter. The particle mass is placed in a mold and pre-pressed to form a shape or mat. The pre-pressed shape or mat is then placed in a press preheated to about 250xc2x0 F. and the press activated to compress the mat for approximately four (4) minutes. The product is then removed and allowed to cool.
The finished product is a board possessing a surface which is smooth, has a coated appearance and beads water. If trimmed at the edges, the internal structure appears homogeneous and does not chip or flake. Inspection of the product after formation reveals complete bonding of the components. Unlike conventional engineered wood products (even those formed from digested fibers), the product of the invention is non-stratified and homogeneous in consistency throughout the entire body formed. It is extremely difficult to distinguish the individual shape of any initial particle. Intentional fracture of boards formed from rice hulls has demonstrated complete lack of layers of stratification of plant matter.
The following examples are illustrative of the process of the invention used to form board products but the invention is not limited to the particular embodiments described.